This invention relates to a video signal analog-to-digital converter for an image display apparatus, in which an upper and a lower potential level are variable so as to determine a gradation range in which the brightness of an image is controlled according to the varying average level of a television video signal.
Recently, liquid crystal television receivers, which use a liquid crystal display panel instead of a cathode-ray tube, have been developed and made known to the public as small, portable television sets. It is generally agreed that the brightness of the image displayed on the liquid crystal display panel can be adequately controlled in 16 gradations.
For providing these 16 different gradations, respective gradation signals are applied to a predetermined electrode provided in the liquid crystal display panel. The gradation signals are obtained by sampling the television video signal, for example, 160 times in a predetermined sampling gate time, and obtaining n-bit, for instance, 4-bit, code signals through analog-to-digital conversion of the sample signal. If 16 4-bit code signals of "0000" to "1111" are obtainable, the brightness can be controlled in 16 gradations between the two extremeties.
In the meantime, the television video signal must be brightness-controlled in 16 gradations from the upper reference potential corresponding to a white level to the lower reference potential corresponding to a black level. The level of the actual video signal, however, varies only in the range toward the upper reference potential when the image is very bright or in the range toward the lower reference potential when the image is very dark. Therefore, of the 16 gradations that are available for image control, only about 10 gradations, for example, are truly effective, so that the range of contrast control is narrowed to that extent.
Moreover, to increase the number of gradations to 32, 64, etc. thereby to improve contrast, an increased number of comparators must be provided in the analog-to-digital converter. This complicates the construction and increases the cost.